The efficient use of natural resources is desirable for economic, ecological and political reasons. Through improved use of renewable resources, we can extend the existence of our non-renewable resources and generate less waste in both production of goods and disposal of goods through recycling of “green” materials. Lignocellulosic biomass is a most readily available renewable natural resource and provides the two most abundant organic compounds on earth in lignin and cellulose, with the other major components including hemicellulose. The usefulness of cellulose has long been recognized, primarily in paper making, but the possibilities of lignin development have only recently begun to be realized. Lignin is a generic term for a complex polymer of aromatic alcohols that varies somewhat between plant species. It is an integral part of the cell wall that covers and protects the cellulose and hemicellulose and presents many challenges to the successful recovery of the biomass components. In order to successfully utilize the components of lignocellulosic biomass, the individual components must first be separated and recovered through pretreatment.
Treatment of lignocellulosic biomass (for instance, wood) via the kraft (or sulfate) process has long been carried out with the primary aim of separating the cellulose in the form of pulp for use in paper formation, in forming derivative polymeric products such as cellophane and rayon, and more recently in the formation of biofuels. The other components of the lignocellulosic biomass (lignin, hemicellulose, etc.) are separated as by-products in the alkaline pulping liquor, called black liquor in the kraft process. The black liquor has historically been recovered so that the sodium and sulfur within can be recovered and used to regenerate the catalysts (sodium hydroxide and sodium sulfide) required in the pulp-making process. Of equal importance, the black liquor is used as an energy source in the kraft process. This recovery process is highly efficient, but provides little in the way of additional value-added products. Other pretreatment processes for obtaining cellulosic pulp from a lignocellulosic biomass, such as the soda and sulfite processes, can also be used to separate the cellulose from the hemicellulose and lignin by-products that remain in an alkaline liquor.
The lignin component of lignocellulosic biomass presents great possibilities for the development of value-added products, as it is unique among renewable biopolymers in having a significant aromatic character and high energy density. For instance, lignin presents possibilities as a green replacement for fossil fuels to provide raw materials for the formation of polymers and other chemicals as well as biofuels and other chemical applications.
Unfortunately, current methods for recovering lignin from lignocellulosic biomass either are prohibitively expensive or provide a product that is too high in impurities to be useful for processing into many value-added products. For instance, the alkaline liquor stream from pulp formation processes represents a huge potential source of lignin (approximately 50 million tons per year), but includes a very high content of ash and metals (mainly sodium and potassium). Other separation and purification methods have been developed to treat lignocellulosic biomass, including acidic treatment (sulfuric acid, oxalic acid, peracetic acid, acetic acid), alkaline treatment (sodium, potassium, calcium, and ammonium hydroxides), organosolv treatment (ethanol, methanol, acetone, or ethylene glycol in combination with water), oxidative delignification (hydrogen peroxide, ozone, oxygen or air), biological methods (cellulolytic organisms such as filamentous fungi), and microwave irradiation. Unfortunately, these methods are either prohibitively expensive, presenting serious issues with regard to scale-up, and/or they provide a lignin product with less-than-desirable physical properties or an unacceptably high impurities content. For instance, even upon further treatment of black liquor by use of a lignin purification process, the metals content of the purified lignin thus produced will generally be greater than 2000 parts per million (ppm) and the ash content will be 1-4% or more, both of which are too high for successful downstream utilization of the lignin for high-value applications. Even those very expensive processes that are able to produce a high-purity lignin, such as organosolv processing, cannot be used to isolate a lignin fraction whose molecular weight is both well-defined and controllable, which would be of great benefit for the further development of lignin-based products.
What is needed in the art is a lignin recovery method that can provide a high-purity lignin with a well-defined, controlled molecular weight in an efficient, economical fashion.